Under the leadership of a government agency, the EPA, and with the aid of modern vehicle electronics, most of the pollution from automobile exhausts has been eliminated. In a well-running car, engine fuel is burnt efficiently, producing in the process the normal exhaust gases.
In an engine that misfires, however, unburnt fuel is expelled as well, contributing measurably to atmospheric pollution. In addition, of course, misfiring causes a significant loss of power of the engine, and may result in severe damage to the engine.
A means of detecting misfiring in an engine is therefore very desirable, so that corrective action can be taken.
Back pressure in the exhaust system of a car increases with engine speed (measured in revolutions per minute or "rpm") and is relatively constant for a given rpm. For a V-8 engine, as an example, this pressure, measured just before the catalytic converter, goes up to 10 psig at the highest rpm, as shown in FIG. 3 below.
The continuous succession of firings in the car engine produces relatively small pressure pulsations around the back pressure, as shown in FIG. 3. In the event of a misfire the back pressure decreases from lack of enough exhaust from the misfiring cylinder and recovers its normal level when gasses from the cylinder that fires next are forced into the exhaust system, causing much larger pressure pulsations, as shown in FIG. 4. This difference between the amplitudes of normal and misfire pressure pulsations, hereinafter called a pressure discontinuity, forms the basis of the present invention for detection of misfires.
Accordingly there has been a need for a simple, compact, reliable and inexpensive real-time pressure discontinuity analysis system capable of detecting such exhaust-gas pressure abnormalities and alerting either a human operator or/and another automatic correction system to the occurrence of a misfire event.
While relatively expensive alarm systems of this type have been proposed, these tend to be complicated and to operate by indirect sensing methods which require sophisticated data-processing procedures and techniques in order to be of practical utility. For example, the "Method of and System for Detecting Misfire in Internal Combustion Engine", U.S. Pat. No. 4,083,234, issued Apr. 11, 1978 (and assigned to Nissan Motor Co., Ltd. of Japan), involves two separate indirect transducers, namely an acoustic transducer, such as an earphone receiver placed near the output of the engine exhaust gas, together with a rotary electromechanical engine-speed responsive frequency generator which produces a frequency variable proportional to the output speed of the engine. Furthermore, the signals from these two separate transducers must be processed by a fairly complicated electromechanical frequency analyzer (involving four separate band-pass filters, and two rotary contact mechanical elements operated at variable frequencies dependent upon engine speed).
It might be possible to apply the output of pressure transducers to suitably adjustable low-pass filters, having a high-frequency cut-off equal to the lowest firing frequency to be attenuated and a high-frequency band-pass maximized to the highest misfire frequency to be detected. Unfortunately the resultant filter design can be optimized only for a given engine rpm, and the development of an adaptive filter, whose frequency-domain shape varies with the engine rpm, though possibly realizable, is not believed to be the most practical or cost-effective approach.
Accordingly there has been a need for a more direct pressure-discontinuity analysis system, which utilizes only one transducer, namely a pressure transducer, and which operates successfully regardless of the values of other related variables (such as engine speed, in the present case of automobile engine misfiring detection systems), such as can be provided by a fixed-parameter passive RLC (Resistance, Inductance, Capacitance) analog circuitry, or a modified circuit of this type containing active elements whose characteristics are fixed independently of such external variables as engine speed.
Also there has been a need for an analysis system which does not include any rotating mechanical parts, and whose mechanical aspects are limited to the simple flexing of a diaphragm of low mechanical hysteresis, which simplification greatly improves reliability and extends lifetime durability, without need of scheduled service, and essentially eliminates the possibility of breakdown-mandated repairs.